1. Field of the Invention.
This invention relates in general to an inter-repeater backplane and method, and more particularly, to an inter-repeater backplane that uses a modified IEEE 802.3 repeater state machine in conjunction with a backplane state machine to operate in either the synchronous or asynchronous mode and which does not require external bus arbitration circuits to control access to the backplane thereby facilitating the removal and replacement of repeater circuits during operation.
2. Description of Related Art.
The communications industry has experienced phenomenal growth over the past several years. Much of this growth has occurred in the area of telecommunications involving computers and computer generated or stored data. The exchange of messages and data has been facilitated by the advent of local and metropolitan area networks. Remotely located computer users communicate over the local and metropolitan area networks to access data and to communicate with other remote computer users.
In such networks, nodes are connected to a bus and have multiple, or concurrent access to the communications medium. Control techniques are used to allow access to the communication medium and to resolve contention between the various users. Typically, a carrier sense, multiple access with collision detection (CSMA/CD) scheme is used. With CSMA/CD a node listens for activity and begins sending message packets when the node determines that no activity is occurring over the network. Sometimes simultaneous transmission by multiple nodes occurs. This results in collisions between the different message packets. When a node detects a collision, a signal is sent over the network to the other transmitting nodes. The affected nodes terminate their transmissions and probabilistically reschedule their next attempt to transmit.
The standard for Local and Metropolitan Area Network technologies is governed by IEEE Std. 802-1991. IEEE Std. 802-1991 describes the relationship among the family of 802 standards and their relationship to the ISO Open System Interconnection Basic Reference Model and is herein incorporated by reference. IEEE Std. 802.3-1991 defines the standards for a bus utilizing collision sense multiple access/collision detection (CSMA/CD) as a data link access method and is herein also incorporated by reference.
Typically, unshielded twisted pair cables or existing telephone wiring is used as the transmission medium to provide an economical solution to networking. However, the attenuation of signals transmitted over unshielded twisted pair cables increases as the distance between data terminal equipment becomes greater. Thus, repeaters are inserted in the twisted pair cables to facilitate greater distances.
To increase the number of data channels, repeater manufacturers have designed repeater hardware which can be cascaded together into a single hub. However, state machine information must be passed between the repeaters over a inter-repeater backplane in order to allow the repeaters to behave as a single hub.
The transmission of data with a repeater system may be accomplished either synchronously or asynchronously. Repeaters have previously operated in either the synchronous or asynchronous mode but not both. For example, Advanced Micro Devices' Am79C981 sends data over the backplane synchronous to the master clock of the Am79C981. Similarly, the AT&T T7201 Multi-port Repeater in combination with the At&T T7202 Smart Hub Controller sends data over the backplane synchronous to the master clock of the chip.
In contrast, National Semiconductor's DP83950A Repeater Interface Controller and the DP83955/DP83956 Lite End Repeater Interface Controller devices (RIC and LERIC respectively) operate in the asynchronous mode when communicating over the backplane. In asynchronous mode the recovered clock is used to clock data to adjacent chips. The asynchronous character of the data transmission derives from the fact that the recovered clock is asynchronous to the system or local clock and therefore requires some form of timing instructions distinct from the system clock.
In addition to the limitations concerning the mode of data transmission, prior art repeaters have required arbitration to control the inter-repeater bus. Repeaters integrated into a single modular hub could not be removed and/or plugged in without the additional circuitry to take care of bus arbitration or the removal of a signal contained therein. Accordingly, boards or repeater chips which were hot-swapped in or out caused the hub system to pass errors, crash, or hang up.
For example, AT&T's MPR2 and SHC chips integrate repeater functions specified by Section 9 of the IEEE 802.3 Standard and twisted-pair transceiver functions complying with the 10Base-T standard. AT&T's MPR2 and SHC have an output pin (ACTIVE) which goes high when any input to the chip is detected. An expansion controller for the chip must be built from discrete logic to monitor the ACTIVE pin from each of the chip comprising the hub. The expansion controller is also needed to pass PORTM, PORTN, and collision information between the repeater chips on the hub. The arbitration scheme controls which chip is controlling the bus at any given time and how collision events are handled.
Advanced Micro Device's IMR+ repeater chip also requires arbitration to control the inter-repeater bus. Advanced Micro Device's IMR+ has an output pin REQ(bar) which goes low when any input to the chip is detected. An expansion controller for the chip must be built from discrete logic to monitor the REQ(bar) pin from each of the chips comprising the hub. The expansion controller is also needed to pass PORTN between the repeater chips. The arbitration scheme controls which chip is controlling the bus at any given time.
National Semiconductor's RIC and LERIC chips require arbitration to control the inter-repeater bus. National Semiconductor's RIC and LERIC chips are more seamless than the repeaters designed by AT&T and Advanced Micro Devices. Nevertheless, to prevent crashes, hang-ups, and to pass error messages, National Semiconductor's RIC chip requires a priority assignment for each of its 13 ports. The top and bottom of this chain is available to the user so that for a multiple chip hub, there will be a daisy chained priority assignment where the AUI port on the first chip has the highest priority and twisted pair port number 12 on the last chip has the lowest priority.
For small configurations, the RICs arbitration scheme is easy to implement. The chip's ACKI(bar) and ACKO(bar) pins can be daisy chained directly, input to output. However, for larger systems the National Semiconductor RIC chip runs into problems with its port arbitration scheme. If a system backplane is used with each RIC placed on a plug-in PC board, the user is forced to use external logic to perform arbitration. The alternative is the already discussed daisy chain arbitration which will disable all the ports lower in the chain if a higher priority board is removed. Further, PORTM and PORTN need this arbitration scheme to be passed between the repeater chips and interpreted correctly.
There is a need, therefore, for a backplane which facilitates both synchronous and asynchronous modes and which allows seamless integration of multiple repeaters into a single hub wherein each repeater may be swapped out without causing the backplane to crash, hang-up or pass error messages. There is also a need for an inter-repeater backplane which does not require additional drivers or external glue logic to implement a multiple board system design.